1. Field of the Invention
The present invention relates to a PLL control circuit of an optical disc apparatus and a recording medium having recorded thereon a program for controlling the optical disc apparatus.
2. Description of the Related Art
Currently, optical disc apparatuses are used which reproduce information by rotating and applying laser beams to optical discs (e.g., CD (Compact Disc), DVD (Digital Versatile disc)). Some optical disc apparatuses have PLL (Phase Locked Loop) circuits to generate a clock (hereinafter, a reproduction clock) for the reproduction process and the rotation control of the optical discs. Describing with reference to FIG. 7, first, the optical disc apparatus performs photoelectric conversion of the reflected light of the laser beam applied to a recording surface of an optical disc to generate an RF (Radio Frequency) signal. The RF signal is binarized at a slice level defined by feedback control of an SLC (Slice Level Control) circuit 101 to generate a binarized signal, which is output to a PLL control circuit 100. The PLL control circuit 100 includes a phase comparison circuit 102, a 1/n division circuit 103, a charge pump circuit 104, an LPF (Low Pass Filter) 105, a VCO (Voltage Controlled Oscillator) circuit 106, etc., and uses feedback control consisting of operation of each circuit to generate the reproduction clock which synchronizes the phase of the frequency signal with the phase of the binarized signal. A decoder 107 at the subsequent stage performs a decode process for the binarized signal based on the reproduction clock to reproduce information in good condition. Hereinafter, locking of the PLL control circuit 100 indicates a state that the PLL control circuit 100 synchronizes the phase of the frequency signal with the phase of the binarized signal to generate the reproduction clock.
By the way, so-called defects such as scratches and dusts may be formed due to impacts or rough handling on the recording surface of the optical disc and the incidence surface where the laser beam is applied. When the laser beam is applied to the defects, the RF signal has a waveform with varied amplitude due to the variation of the light amount of the laser beam or the reflected light, as shown by the RF signal (period T12) of FIG. 8, for example. Since the amplitude of the RF signal is varied, the slice level of the SLC circuit 101 becomes unsteady and, as a result, the binarized signal becomes an unsteady signal, which is input to the PLL control circuit 100. Therefore, the phases of the binarized signal and the frequency signal may become asynchronous, and the locking of the PLL control circuit 100 may be released (hereinafter, unlocking of the PLL control circuit 100). This RF signal may affect signals (such as a focus error signal and a tracking error signal) generated by the optical disc apparatus for various servo controls (focus control and tracking control) and may generate malfunctions of the optical disc apparatus. Therefore, the optical disc apparatus may include a defect detection circuit 108 for detecting the RF signal affected by defects. The PLL control circuit 100 may include a lock determination circuit 109, a CP (Charge Pump) boost control circuit 110, and a timer 111 to relock the PLL control circuit 100 regardless of the phase comparison of the phase comparison circuit 102 when the laser beam is no longer applied to the defect due to the rotation of the optical disc and the binarized signal processable for reproduction is input again.
With reference to FIG. 8, description will be made of the detection of the RF signal affected by defects and the relocking of the PLL control circuit 100. The RF signal until t10 and after the period T12 shows an example of the RF signal that is not affected by defects.
The defect circuit 108 detects the RF signal affected by defects, for example, by determining whether a difference between the peak level and the bottom level of the RF signal becomes less than a predetermined level. When it is determined that the difference between the peak level and the bottom level of the RF signal becomes less than a predetermined level, the defect detection circuit 108 outputs a high-level defect signal (t11). For example, the optical disc apparatus suspends various servo controls based on the rising edge of the defect signal. As a result, the malfunctions, etc., due to the defects can be avoided. The optical disc apparatus resumes various servo controls based on the falling edge of the defect signal.
On the other hand, the lock determination circuit 109 compares the phase of the binarized signal with the phase the frequency signal to determine the locking or unlocking of the PLL control circuit 100. When it is determined that the PLL control circuit 100 is unlocked (t12), the lock determination circuit 109 outputs a signal indicating the determination result to the CP boost control circuit 110. The CP boost control circuit 110 allows the timer 111 to start clocking based on the signal from the lock determination circuit 109 to determine whether a predetermined period T11 has elapsed. If it is determined that the clocking of the timer 111 attains the period T11, the CP boost control circuit 110 transmits to the charge pump circuit 104 a high-level signal (hereinafter, a CP control signal) for synchronizing the phases of the binarized signal and the frequency signal regardless of the phase comparison of the phase comparison circuit 102 (t13). The charge pump circuit 104 performs so-called boost operations that increases a voltage output to the LPF 105 based on the CP control signal. The LPF 105 outputs to the VCO circuit 106 a control voltage (hereinafter, a VCO control voltage) acquired by smoothing the output voltage from the charge pump circuit 104. As shown in FIG. 8, the VCO control voltage is repeatedly increased in accordance with the above boost of the charge pump circuit 104 to synchronize the phases of the binarized signal and the frequency signal. The VCO circuit 106 generates a frequency signal corresponding to the level of the VCO control voltage and outputs the signal to the 1/n division circuit 103. The 1/n division circuit 103 outputs to the phase comparison circuit 102 and the lock determination circuit 109 a frequency signal acquired by dividing the frequency signal from the VCO circuit 106 into 1/n. When the binarized signal processable for reproduction is input again, to quickly synchronize the phases of the binarized signal and the frequency signal (to relock the PLL control circuit 100), the charge pump circuit 104 is preliminarily boosted to increase the output voltage of the charge pump circuit 104 after the predetermined period T11 has elapsed.
When the laser beam is no longer applied to the defect due to the rotation of the optical disc, the RF signal processable for reproduction is input to the SLC circuit 101 again. This RF signal is binarized to generate a binarized signal, which input to the phase comparison circuit 102. The phases of the binarized signal and the frequency signal are quickly synchronized by the feedback control of the PLL control circuit 100 and the above boost of the charge pump circuit 104 (t14). As a result, the information can be reproduced from the optical disc without impairing the binarized signal. If it is determined that the PLL control circuit 100 is relocked from the phase comparison of the binarized signal and the frequency signal, the lock determination circuit 109 transmits to the CP boost control circuit 110 a signal for stopping the output of the CP control signal from the CP boost control circuit 110 (t15).
Such a conventional optical disc apparatus is disclosed in Japanese Patent Application Laid-Open Publication Nos. 10-208244 and 10-112141, for example.
However, in the conventional PLL control circuit 100, the start of the boost of the charge pump circuit 104 is controlled by time management based on the unlocking of the PLL control circuit 100 and, therefore, variations may be generated in the boost period of the charge pump circuit 104.
Specifically, for example, if the period T12 becomes long relative to the period T11, the period T11 may become relatively short which is a period until the boost of the charge pump circuit 104 and the boosting period of the charge pump circuit 104 may become long. Therefore, power consumption may be increased because the boost period of the charge pump circuit 104 becomes long. Since the boost is performed while the slice level of the SLC 101 is not determined, the VCO control voltage becomes more unsteady, resulting in delay in the phase synchronization. Alternatively, if the period T12 is short relative to the period T11, the boost period of the charge pump circuit 104 may become short in the period T12. Therefore, although the binarized signal processable for reproduction is input to the PLL control circuit 100, the synchronization of the phases of the binarized signal and the frequency signal may be delayed and the optical disc apparatus may not quickly perform the information reproduction process. As a result, performance may be deteriorated in the information reproduction process in the optical disc apparatus.